Method for forming a retroreflective sheeting

ABSTRACT

The method for forming a retroreflective sheeting includes providing a first layer and forming a prism array on the first layer. A second layer is applied to the prism array. The first thermoplastic layer is welded to the second thermoplastic layer, while applying a die to the layers to dislocate a portion of the prism array, thereby allowing the first layer to be bonded to the second layer at the portion.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.08/976,095, U.S. Pat. No. 6,039,909 filed Nov. 21, 1997, which is acontinuation of U.S. application Ser. No. 8/443,836, U.S. Pat. No.6,143,224 filed May 18, 1995, the entire teachings of both beingincorporated herein by reference.

BACKGROUND OF THE INVENTION

Retroreflective materials are employed for various safety and decorativepurposes. Particularly, these materials are useful at night time whenvisibility is important under low light conditions. With perfectretroreflective materials, light rays are reflected essentially towardsa light source in a substantially parallel path along an axis ofretroreflectivity. For many applications, perfect retroreflectivity isnot required. Rather, a compromise is required in which a cone ofdivergence is provided which permits a degree of divergence whichenables enough divergent light to strike the viewer's eye, yet not somuch that the intensity of the reflective light at the viewer's eye isunduly diminished. Under circumstances where the only source ofillumination is the headlights of an automobile on an unlit road, theability to retroreflect a cone of divergence to the eye of the driver isimportant for safety reasons.

Many types of retroreflective material exist for various purposes. Theseretroreflective materials can be used as reflective tapes and patchesfor clothing, such as vests and belts. Also, retroreflective bands canbe used on posts, barrels, traffic cone collars, highway signs, warningreflectors, etc. Retroreflective material may be comprised of arrays ofrandomly oriented micron diameter spheres or close packed cube-corner(prismatic) arrays.

Cube-corner or prismatic retroreflectors are described in U.S. Pat. No.3,712,706, issued to Stamm on Jan. 23, 1973. Generally, the prisms aremade by forming a master negative die on a flat surface of a metal plateor other suitable material. To form the cube-corners, three series ofparallel equidistance intersecting V-shaped grooves 60 degrees apart areinscribed in the flat plate. The die is then used to process the desiredcube-corner array into a rigid flat plastic surface.

When the groove angle is 70 degrees, 31 minutes, 43.6 seconds, the angleformed by the intersection of two cube faces (the dihedral angle) is 90degrees and the incident light is reflected back to the source. Forautomobile headlight reflectors, the dihedral angle is changed so thatthe incidental light is reflected nonorthogonally towards the driverinstead of the source.

The efficiency of a retroreflective structure is a measure of the amountof incidental light returned within a cone diverging from the axis ofretroreflection. Distortion of the prismatic structure adversely effectsthe efficiency. For instance, if the prismatic structure is formed of athermoplastic, the structure can distort if it is overstressed, therebydecreasing the efficiency of the retroreflective structure. One solutionis to form a prismatic structure with a hard polymer. However, if thesupport sheeting is formed of a thermoplastic, a suitable weld isdifficult to form between the thermoplastic sheets, thereby allowing theformed structure to tear easily along the weld. Furthermore, cube-cornerretroreflective elements have low angularity, i.e., the element willonly brightly retroreflect light that impinges on it within a narrowangular range centering approximately on its optical axis. Lowangularity arises by the inherent nature of these elements, which aretrihedral structures having three mutually perpendicular lateral faces.The elements are arranged so that light to be retroreflected impingesinto the internal space defined by the faces, and retroreflection of theimpinging light occurs by internal reflection of the light from face toface of the element. Impinging light that is inclined substantially awayfrom the optical axis of the element (which is the trisector of theinternal space defined by the faces of the element) strikes a face at anangle less than its critical angle, thereby passing through the facerather than being reflected.

Further details concerning the structures and operation of cube-cornermicroprisms can be found in U.S. Pat. No. 3,684,348, issued to Rowlandon Aug. 15, 1972, the teachings of which are incorporated by referenceherein. A method for making retroreflective sheeting is also disclosedin U.S. Pat. No. 3,689,346, issued to Rowland on Sep. 5, 1972, theteachings of which are incorporated by reference herein. The disclosedmethod is for forming cube-corner microprisms in a cooperativelyconfigured mold. The prisms are bonded to sheeting which is appliedthereover to provide a composite structure in which the cube-cornerformations project from one surface of the sheeting.

SUMMARY OF THE INVENTION

A method for forming a retroreflective sheeting includes the steps ofproviding a first thermoplastic polymer layer and forming a rigid prismarray on the first thermoplastic polymer layer. A second thermoplasticlayer is applied to the prism array. The first thermoplastic layer iswelded to the second thermoplastic layer while applying a die to thefirst and second thermoplastic layers to dislocate a portion of therigid prism array, thereby allowing the first thermoplastic layer to bebonded to the second thermoplastic layer at said portion.

The retroreflective structure includes a first thermoplastic polymerlayer and a rigid prism array attached to the first thermoplasticpolymer layer. A second thermoplastic layer has a portion of the secondthermoplastic layer welded to the first thermoplastic layer through theprism array.

The invention has many advantages including providing a high strengthweld which renders the sheeting resistant to tearing at the welds. Theelements of the prism array are formed of a rigid polymer which allowthe elements to retain their optical characteristics better thannonrigid elements after having been subject to stretching. Clothingapparel, such as running suits, can have the retroreflective structureattached to an outer surface of said apparel to enhance visibility ofthe wearer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of the first embodiment of aretroreflective structure of the present invention prior to welding.

FIG. 2 is a cross-sectional view of the first embodiment of theretroreflective structure of the present invention after welding.

FIG. 3 is a partial top view of the first embodiment of theretroreflective structure of the present invention after welding.

FIG. 4 is a partial top view of a second embodiment of theretroreflective structure of the present invention.

FIG. 5 is a cross-sectional view of a third embodiment of a mold forforming a retroreflective structure of the present invention.

FIG. 6 is a cross-sectional view of the mold and retroreflectivestructure with the mold extending into the backing film.

FIG. 7 is a cross-sectional view of the mold and retroreflectivestructure with the mold having been extended through the backing filmand into the base film.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The features and other details of the method and apparatus of theinvention will now be more particularly described with reference to theaccompanying drawings and pointed out in the claims. The same numeralpresent in different figures represents the same item. It will beunderstood that the particular embodiments of the invention are shown byway of illustration and not as limitations of the invention. Theprincipal features of this invention can be employed in variousembodiments without departing from the scope of the invention.

Retroreflective structure 10, as shown in FIG. 1, has a base film 12that is comprised of a transparent thermoplastic film, such as polyvinylchloride, polyvinylidene chloride, urethane films, polyfluorocarbonpolymers, etc., which has a low temperature of distortion. In oneembodiment, a low temperature of distortion is considered to be about180° F. (82° C.). The distortion temperature is the temperature at whicha polymer melts and begins to flow. In another embodiment, thethermoplastic is an ethylene-tetrafluoroethylene copolymer. Base film 12can be transparent to visible light and can be either clear or colored.An example of a suitable base film 12 is a polyvinyl chloride filmavailable from Renoliot Corp. under the trademark Renoliot™ H1W series.Base film 12 can have a thickness in the range of between about 0.003and 0.02 inches (0.0076 and 0.051 cm). In a preferred embodiment, thethickness is in the range of between about 0.0085 and 0.022 inches(0.022 and 0.056 cm). The selected thickness is dependent upon themethod of fabrication, such as radio high frequency welding orultrasonic welding, the thermoplastic selected, and the characteristicsdesired for the retroreflective structure.

The prism array 14, which can include retroreflective cube-comer prismelements 16, is formed on the base film 12. Prism array 14 has a windowside 18 and facet sides 20 and is attached on window side 18 to the basefilm 12. Prism array 14 is formed of a transparent polymer that has ahigh temperature of distortion, which is the temperature at which thepolymer melts and flows. The temperature of distortion for the polymerin the prism array 14 is sufficiently higher than the temperature ofdistortion for the polymer in the base film, thereby allowing thepolymer in the base film to melt before the polymer in the prism arraywhen exposed to a heating source. For example, the difference intemperatures of distortion for the two polymers is preferred to begreater than about 100° F. (56° C.) and preferably greater than about150° F. (83° C.). In one embodiment, the temperature of distortion ofthe polymer of prism array 14 is about 350° C. (177° C.). After beingformed, the polymer is substantially rigid at room temperature, which isdefined as being substantially inflexible. This rigidity of the polymerin the prism array allows the prism elements to retain their opticalcharacteristics. The prism array polymer can also be nonextensible,which is defined as not being capable of being substantially stretchedwithout breaking. The polymer is selected from a wide variety ofpolymers which include the polymers of urethane, acrylic acid esters,cellulose esters, ethylenically unsaturated nitrites, hard epoxyacrylates, etc. Other polymers include polycarbonates, polyesters andpolyolefins, acrylated silanes, hard polyester urethane acrylates.Preferably, the polymer can be cast in a prismatic mold with a monomeror oligomer polymerization initiated by ultraviolet radiation.

The prisms elements 16 of the prism array 14 can be cube-comer in shapeand have a length along each cube-side edge in the range of betweenabout 0.004 and 0.02 inches (0.01 and 0.051 cm). In one embodiment, eachcube-side edge has a length of about 0.006 inches (0.015 cm).Preferably, each cube-side edge has a length of between about 0.004 and0.008 inches (0.01 and 0.02 cm).

The thickness of prism array 14 at valley 22, where the rigid prismelements intersect, is sufficiently thin so that the prism array 14 cancrack and split along the valleys 22 when a minimal force is applied toretroreflective structure. In one embodiment, the thickness of prismarray 14 is in the range of between about 0.0028 and 0.009 inches (0.007and 0.023 cm).

The base film 12 provides a substrate for prism array 14 to provide asmooth surface upon which the prism elements 16 can attach, preferablyto the window side 18 of the prism elements 16. The prism array 14 canbe laminated to the base film 12 with a transparent adhesive.Alternatively, the prism array 14 can be cast directly onto the basefilm 12.

Backing film 24 is placed on the facet side 20 of the prism array 14.The backing film 24 can be formed of a thermoplastic having atemperature of distortion that is about the same as base film 12. Forinstance, backing film 24 can be formed from a thermoplastic, such as apolyvinyl chloride, polyvinylidene chloride, urethane films,polyfluorocarbon polymers including an ethylene-tetrafluoroethylenecopolymer, etc., which has a low temperature of distortion. Thethermoplastic of backing film 24 is transparent to visible light and iseither clear or colored. In a preferred embodiment, the base film 12 andbacking film 24 both include polyvinyl chloride. Backing film 24 canhave a thickness in the range of between about 0.005 and 0.02 inches(0.013 and 0.051 cm).

Unlike standard vinyl retroreflective sheeting, the present structure isnot constructed purely of vinyl. While both materials have dipolestructures and are susceptible to high frequency (dielectric) action,their heat distortion temperatures can be more than about 190° F. (106°C.) apart. The heat distortion temperature is the temperature at whichthe polymer begins to flow. The thermoplastic base film 12 has a heatdistortion temperature of about 180° F. (82° C.), while the nonvinylmaterials can have a distortion temperature of about 350° F. (177° C.).

The prism array 14 appears to act as barrier to the welding of the firstlayer, base film 12, to the second layer, backing film 24. An effectiveway to weld and form the sheeting is to move the prisms out of the wayand allow the thermoplastic in the base film 12 and the backing film 24to form a bond. The pyramidal shape of the prisms promotes instabilityand if the base film 12 can be softened, the prisms will move or tumbleout of the way.

High frequency welding is the joining of two thermoplastic surfaces bymelting under heat and pressure, which is brought about by molecularfriction. Molecules within the material are subject to internal stressescaused by an electrical field alternating in polarity several milliontimes per second. When the heat exceeds the melting point (distortiontemperature) of the base film 12 under pressure, the two surfacescollapse and meld, thus forming a joint.

The welding can be performed by the use of radiation frequency ordielectric sealing equipment. Suitable welding equipment includesequipment sold under the tradenames, Thermatron, Kosmos, Kiefel andCallaghan. The equipment operates on the principle of a generatorproducing radio energy at a frequency of about 27.12 MHz. As shown inFIG. 2, the press has an upper platen 26 with a die 28 and a lowerplaten 30. Although not shown, lower platen 30 can also have a die.Alternatively, only the lower die can have a die. In the raised or upposition, the base film 12 with prism array and the backing film 24 arelaid against the die 28 that has a seal pattern 32. Upon closure of thepress, high frequency radio energy is applied, causing the polarmolecules in the base film 12 and the backing film 24 to become agitatedand heated to their temperature of distortion and melting point, whilebeing sufficiently low as not to distort the polymer of the prism array14.

The die 28, which is pressed against the two films, contains lands 34and edges 35. The prism elements 16 move and allow the two films to meetand thus meld or join together. The amount of this welding is controlledby time, temperature and radio frequency power level. For example, informing a polymer chloride sheeting with acrylated-epoxy prisms and areinforced polyvinyl chloride backing at a rate of six feet per second,platen pressure can be about 50-60 psi and a platen temperature of about115-125° F. (46-52° C.). The radio energy can be about 9.5 kWatts. Theplaten current can be about 1.5 amps, and a grid current can be about0.9 amps. Pressure can be applied by the heated platens for a presealstep of about one second prior to exposure to the radio frequency, thenfor a sealing step of about 2.4 seconds while the sheeting and prismsare exposed to the radio energy and then for a cooling step afterexposure to the radio energy for about one second.

In a preferred embodiment, the die employed is etched brass. The die canalso be formed of magnesium, etched steel, copper or any other suitabledie making material known in the art. The die has an outer tear seal,which is a sharp edge. This allows the formed retroreflective structureto be removed from the skeleton, or waste, around the perimeter of theretroreflective structure 10. For a magnesium die, the inner seal widthsare referred to as 9 point line weight (0.013 inches), which is anindication of their etched width. However, the line weights can rangefrom about 0.1 to 3.0 points, which is about 0.001 to 0.042 inches.These inner seals are then machined down about 0.014 inches (0.036 cm)below the height of the tear seal in order not to tear the inner cells.In the case of a brass die, the inner seals are kept to a height ofabout 0.008 inches below the surface of a tear seal. The brass die caneither be machined from a solid piece of brass or constructed frompieces of brass rule.

As shown in FIG. 3, the inner seal on the die 28 form a plurality ofcells 36 with a multi-course, hatched pattern perimeter, which whenapplied to the base film 12 (first thermoplastic polymer layer), theprism array 14, and the backing film 24 (second thermoplastic polymerlayer) cause a portion of the prism array 14 (not shown in FIG. 4) todislocate, thereby allowing the first thermoplastic layer to bond to thesecond thermoplastic layer. The cells 36 are preferably a half to oneinch in length and width. The perimeters of the cells 36 are formed ofmultiple courses 38 with hatches 40. A product, such as a tape, has anedge that is generally sealed on both sides having a width of about 0.25inches (0.64 cm). This seal, which is air tight, is made in a certainpattern, and a brick type having several layers wide along the edges hasbeen found to be superior. Other patterns can include squares, diamonds,triangles, etc. In one embodiment, the pattern is a plurality of offsetrows of rectangles 42 (a brick pattern), wherein the rectangles areabout 0.0625 by 0.125 inches (0.16 by 0.32 cm). The lands 34 of the die28 can be flat thus forming a uniform indention for each rectangle.Preferably, there are three rows of offset rectangles. In anotherexample, a plurality of offset rows of squares can form the pattern. Inanother example, as shown in FIG. 4, the rows can be formed oftriangles. In one embodiment, the triangles have a height of about 0.125inches (0.32 cm).

The multi-course, hatched pattern provides strength to the sealedproduct. It also provides a pocket in each cell 36, so that if onesection of a product should fail by tearing, ripping or leaking, theremainder of the retroreflective structure is still operating bothcosmetically and reflectively.

Another important consideration is the thickness of the base film 12.The base film 12 should have a thickness in the range of between about0.0085 and 0.011 inches (0.022 and 0.028 cm) in thickness. If the basefilm 12 is too thin, the material does not weld together with muchstrength. If it is too thick, the prisms will not move out of the way,and the material will not melt together.

In another embodiment, as shown in FIG. 5, a bar seal die land can bemodified from a flat surface to a “M” shape 48. This design promotesmovement of the prisms and allow the vinyl substrates to form a weld. Inone embodiment, the height of the prongs from the platen is about atenth of an inch (A) and at an angle α from the platen, which can beabout 120°. Prongs 50 are separated by a distance (B) of about 0.018inches (0.046 cm) and having a depth (C) of about 0.015 inches (0.038cm). Trough 52 can have an angle of about β, which can be about 60°. Thedie has a thickness (D) of about 0.25 inches (0.064 cm). This designcauses the prisms in the array to be moved out of the way, allowing thebase film and backing film to weld together. In FIG. 6, the die seal isshown extending into the backing film 24. In FIG. 7, the die seal hadbeen extended through the base film 12.

Energy can be also supplied by ultrasonic energy, infrared energy orinduction heating for joining the layers. Suitable ultrasonic weldingequipment includes equipment sold under the trade names of Branson andDukane. In a continuous web process, the base film 12, prism array 14and a backing film 24 of about 0.0085 inches (0.022 cm) are broughttogether into a rotating die which has inner seals and a pattern, eitherdiamond or stripped. The tape then proceeds to a second station, wherethe edges are sealed together with a rotating wheel cutter. If the wheelcutter is not rotating, the material does not seal nor can the innerseals and the edge seals be put onto the tape at the same station.Ultrasonic plunge sealers have also been used to seal the edges of apreviously radio frequency sealed product, allowing a cone or ring to bewelded together. For instance, the ultrasonic frequency can be twentykHz at a power level of about 1,000 to 2,000 watts. The retroreflectivesheeting can be formed at a rate of 10 to 40 feet per minute.

The heat in high frequency welding is generated at the mid-point betweenthe upper and lower platens. Since the melting temperature of thepolymer in the prism layer is almost twice as high as the thermoplasticfilms, it is important that the shape of sealing die promote themovement of the prism structure out of the way and permit the twocomponents to heat up and bond together.

Care must be taken to insure that the base film 12, prism array 14 andbacking film 24 interface is at the midpoint of the upper and lowerplatens in order to optimize the heat generated by the high frequencyenergy. For this reason, it is generally better to use a thicker backingmaterial than that is used with all vinyl structures in order to providesufficient vinyl to form a weld.

The seal dwell time and dielectric current settings required to form thewelds are generally 10-20% higher than those used in traditional highfrequency welding. Normal seal dwell times and dielectric settingsshould be used as a base line to begin process optimization. Clampingpressures should be lower than those used in traditional high frequencywelding. High frequency welding machines vary in regard to power outputand pressure from manufacturer to manufacturer and even between machinesof the same model. As a rule, it is prudent to evaluate the performanceof each machine for different dies and backings as settings may varyfrom machine to machine.

Too much clamp pressure or an uneven platen or die or both can result inoversealing. Oversealing can weaken the material around the weld and cancause an uneven surface making printing difficult. Sealing through thebacking can result in a slightly uneven front surface. If the depth ofseal is not too great, if too little pressure is applied, the prisms donot sufficiently move, and this results in no weld at all.

The platen and dies must be as level as possible in order to weldnonvinyl prism sheeting. This is especially true when bar sealing. Downstops should be employed to prevent the die from pushing too far intothe substrates.

Weld strength can be tested by the following methods: Cutting into thematerial and pulling the two substrates apart. The weld is adequate formost applications if either of the films tear before the weld. Soakingthe retroreflective structure in room temperature water for 24 hours.Leakage into any of the cells constitutes a failure. Soaking theretroreflective structure in a water bath with an initial temperature of150° F. (66° C.) and allowing the bath cool to room temperature. Thismethod expands the air in the welded cell, stressing the weld, as thewater cools, water will be pulled into the cell as the air contracts,thereby stretching the material. In general, the most reliable test isto cut into a sample of the material and attempt to separate thesubstrates. If the sealing equipment is properly calibrated, the diesand platens are leveled and the platen temperatures and dielectricsettings are maintained, only periodic testing should be needed toverify the weld performance.

Equivalents

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the spirit and scope of theinvention as defined by the appended claims.

What is claimed is:
 1. A method for forming a retroreflective sheeting,comprising the steps of: a) providing a first light transparentthermoplastic polymer layer; b) forming prism elements in a prism arrayon the first thermoplastic polymer layer, said prism elements beingformed of a polymer material which is light transparent andsubstantially inflexible at room temperature; c) applying a secondthermoplastic layer to the prism array on a side of the array oppositethe first layer; and d) welding the first thermoplastic layer to saidsecond thermoplastic layer, while applying a die to said layers todislocate a portion of the prism elements of the prism array, wherebythe first thermoplastic layer bonds to the second thermoplastic layer atsaid portion, thereby forming the retroreflective sheeting.
 2. Themethod of claim 1 wherein said first thermoplastic polymer layer has athickness in the range of about 0.003 to 0.022 inches.
 3. The method ofclaim 1 wherein said first thermoplastic polymer layer has a thicknessin the range of about 0.085 to 0.02 inches.
 4. The method of claim 1wherein said first thermoplastic polymer layer has a thickness in therange of about 0.085 to 0.011 inches.
 5. The method of claim 1 whereinthe first thermoplastic layer is selected from a group consisting ofpolyvinyl chloride, polyvinylidene chloride, polyurethane andpolyfluorocarbon.
 6. The method of claim 5 wherein the secondthermoplastic layer is selected from a group consisting of polyvinylchloride, polyvinylidene chloride, polyurethane and polyfluorocarbon. 7.The method of claim 1 wherein the rigid prism array is formed of apolymer selected from a group consisting of epoxy polyacrylates,polyurethane, polynitriles, polycarbonates, polyesters and polyolefins.8. The method of claim 1 wherein the rigid, prism array is formed ofmore than one polymer.
 9. The method of claim 1 wherein the die includesa land having at least one flat.
 10. A method for forming aretroreflective sheeting, comprising the steps of: a) providing a firstlayer having a first light transparent thermoplastic polymer; b) formingprism elements in a prism array on the first thermoplastic polymerlayer, said prism elements being formed of a polymer material which islight transparent and substantially inflexible at room temperature andthat has a substantially higher distortion temperature than thedistortion temperature of the first light transparent thermoplasticpolymer layer; c) applying a second thermoplastic polymer layer to theprism array on a side of the array opposite the first layer, wherein thesecond thermoplastic layer has a distortion temperature that is aboutthe same as the first light transparent thermoplastic polymer layer; andd) welding the first layer to the second layer, while applying a die tosaid layers to dislocate a portion of the prism elements of the prismarray, whereby the first layer bonds to the second layer at saidportion, thereby forming the retroreflective sheeting.
 11. The method ofclaim 10 wherein the first thermoplastic polymer layer and the secondthermoplastic polymer layer include the same polymer.
 12. The method ofclaim 10 wherein the first thermoplastic polymer layer includes apolyvinyl chloride.
 13. The method of claim 10 wherein the secondthermoplastic polymer layer includes a polyvinyl chloride.
 14. Themethod of claim 10 wherein the polymer material of the light transparentprism elements includes an epoxy or carbonate.
 15. The method of claim10 wherein the first thermoplastic polymer layer and the secondthermoplastic polymer layer have a heat distortion temperature of about180° F.
 16. The method of claim 10 wherein the polymer material of thelight transparent prism elements has a heat distortion temperature ofabove about 350° F.
 17. The method of claim 10 wherein the die includesa land having at least one flat.